1. Field of the Invention
The present invention relates to a microcapsule-type curing agent, a method for producing the same, a thermosetting resin composition, a prepreg and a fiber reinforced composite material. More particularly, the present invention relates to a microcapsule-type curing agent which can provide a thermosetting resin composition excellent in storage stability at a room temperature and a method for producing the same, a thermosetting resin composition containing the microcapsule-type curing agent which is excellent in storage stability and can provide a cured product good in thermal resistance, and a prepreg and fiber reinforced composite material comprising the thermosetting resin composition.
2. Description of the Prior Art
Since a thermosetting resin is excellent in various properties such as mechanical properties, electrical properties, chemical resistance, thermal resistance and adhesiveness, it is used not only for composite materials but also for various fields such as painting materials, electric and electronic materials and adhesives. Such a thermosetting resin usually starts to react by mixing a resin with a curing agent or a curing-accelerating agent. Therefore, as a thermosetting resin, a two-package type resin system has been generally used, wherein the non-cured resin and the agent are separately preserved and they are measured and mixed immediately before their use. Recently, however, requirements for a one-package type thermosetting resin system, in which a curing agent is pre-mixed with a resin, have increased due to its good handling ability even for various uses.
Particularly, in a fiber reinforced composite material comprising reinforcing fibers such as carbon fibers, glass fibers or aramide fibers and a matrix resin, a prepreg, which is an intermediate material in which a matrix resin is impregnated into reinforcing fibers, is frequently used, and the material is frequently formed by stacking a plurality of prepregs and curing the stacked prepregs. A thermosetting resin, particularly an epoxy resin is frequently used for-the matrix resin. Further, a cyanate ester resin, a maleimide resin and a resin produced by pre-reaction of a cyanate ester resin and a maleimide resin are also used because of their good thermal resistances. As such a matrix resin, mainly a one-package type thermosetting resin system is used, and for example, a resin wherein a latent type curing agent such as dicyandiamide, a hydrazide compound or a boron trifluoride compound is mixed with an epoxy resin is used. However, because such a one-package type thermosetting resin system does not have a sufficient storage stability for a long period, and there is a possibility that the resin and the curing agent react even at room temperature, tack and drape which are important properties of a prepreg decrease. Therefore, a prepreg is used while applying a method for preserving the prepreg in a refrigerator or a method for shortening the period of time for which the prepreg is left at a room temperature before forming. From such present conditions, development of a thermosetting resin composition and a prepreg which are excellent in storage stability at a room temperature is desired.
In order to improve the storage stability of a one-package type thermosetting resin, attempts to prevent reaction of a non-cured resin and a curing agent by forming the curing agent as a microcapsule have been proposed.
JP-B-SHO 54-31468 discloses a method for producing microcapsule-type curing agent whose shell material is formed by an epoxy resin, and describes that the curing agent is useful for a one-package type adhesive. In this method, the capsule is formed by interfacial reaction of a water soluble epoxy resin and an amine compound, but the shell material of the microcapsule is brittle and the shell is easily broken by external pressure to release the curing agent. Therefore, when a resin and the microcapsule-type curing agent are mixed, the capsule is likely to be broken, and as a result, the microcapsule-type curing agent cannot be applied for various uses.
Further, JP-A-HEI 2-292325 and JP-A-HEI 3-220246 disclose microcapsule-type curing agents applicable to epoxy-based adhesives or paints. In these microcapsules, the shell materials are formed from a thermoplastic resin having a specified softening point, and when heated at a temperature higher than the softening point, the capsule is broken and the reaction is initiated. However, because the softening point of the shell material is low and a cured resin contains a thermoplastic resin with a low thermal resistance, there is a problem that the thermal resistance of a cured resin decreases.
Further, the mean particle diameters of these microcapsule-type curing agents are too great to uniformly cure a resin. Furthermore, since the-large microcapsule-type curing agent cannot enter into a gap between reinforcing fibers when applied to a matrix resin for a prepreg, a prepreg excellent in quality cannot be obtained because of, for example, occurrence of non-uniform curing condition when the prepreg is cured, and a composite material obtained by curing the prepreg also cannot have a high thermal resistance.